The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Compression-ignition internal combustion engines operate at lean air/fuel ratios to achieve desirable fuel efficiencies. Lean engine operation may produce oxides of nitrogen (NOx) when nitrogen and oxygen molecules present in engine intake air disassociate in the high temperatures of combustion. Rates of NOx production follow known relationships in the combustion process, for example, with higher rates of NOx production being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. NOx molecules may be reduced to nitrogen and water in aftertreatment devices. Efficacy of known aftertreatment devices is dependent upon operating conditions including operating temperature, which is associated with exhaust gas flow temperatures and engine air/fuel ratio. Aftertreatment devices include materials prone to damage or degradation when exposed to elevated temperatures and/or contaminants in the exhaust gas feedstream.
Aftertreatment systems purify exhaust gases by filtering, oxidizing and/or reducing constituents in an exhaust gas feedstream. Three-way catalytic devices (TWC) oxidize and reduce exhaust gas constituents. NOx adsorbers store NOx, which may be subsequently desorbed and reduced under specific engine operating conditions. Diesel particulate filters (DPF) are able to remove particulate matter in the exhaust gas feedstream through mechanical filtering.
The exhaust gas treatment system of a lean-burn engine typically contains a diesel oxidation catalyst (DOC). When the exhaust gas stream is passed through the DOC it contacts a catalyst material, such as platinum, that is capable of oxidizing CO to CO2, HC to CO2 and water, and NO to NO2. The exhaust gas may then be passed through a selective catalytic reduction (SCR) system located downstream of the oxidation catalyst within the treatment system. An SCR operates by injecting a reductant material, such as ammonia or unburned fuel constituents, into the exhaust gas stream before the exhaust gas is passed over a reduction catalyst material configured to reduce an amount of NO2 to N2 in the exhaust gas in the presence of the reductant additive.
In another approach, an engine that primarily operates in a fuel-lean mode may be controlled to briefly operate in a fuel-rich mode to increase the amount of unburned fuel constituents in the exhaust gas. When the engine is operated in the fuel-rich mode, the fuel constituents in the exhaust gas promote the reduction of NO2 to N2 in the presence of a reduction catalyst. In this treatment method, the exhaust gas is passed in contact with a combination of materials that, when combined in a treatment system, are capable of efficiently reducing NOx to N2. Such a combination is known as a Lean NOx Trap (LNT). A conventional LNT includes a NOx oxidation catalyst, a NOx reduction catalyst, and a NOx storage material to temporarily store, or “trap,” the NOx. LNTs function under cyclical oxidizing and reducing exhaust gas conditions. The cyclical exhaust gas environment is controlled by operating the associated engine in a fuel-lean mode for a major portion of an engine control cycle and in a fuel-rich mode for a minor portion of the cycle. The engine control cycle is repeated, and the controlled cyclical oxidizing and reducing exhaust gas conditions are produced.
Vehicles with an Internal Combustion Engine (ICE) include an exhaust gas treatment system for treating the exhaust gas from the engine. The treatment system typically includes a close-coupled catalytic converter and an underfloor catalytic converter, each of which includes a catalyst that reduces nitrogen oxides in the exhaust gas to nitrogen and water, as well as oxidizes carbon monoxide (CO) and unburnt hydrocarbons (HCs) to carbon dioxide and water. The catalyst may include, but is not limited to, Platinum Group Metals (PGM). The catalyst is not operational until it is heated to a certain temperature, often referred to as the “light-off” temperature. The exhaust gas may be used to heat the catalyst to the light-off temperature for treatment of the exhaust gas.
Low-temperature emission control technologies may include an exhaust gas heater, such as but not limited to an electric heating module, to further heat the exhaust gas to reduce the time to heat the catalyst to the light-off temperature. Including such a device, however, may add significantly to the complexity and cost of the emission control system.